One Exciter For All Standards
The proliferation of digital standards continues to be challenging
— not only for the consumer and receiver manufacturers, but also
for the transmission equipment manufacturer. Indeed, at the time
of writing, there are at least 11 known digital modulation
standards worldwide that are either in use or soon will be. These
include systems specifically engineered for fixed and mobile
applications. Such standards include ATSC, DVB-T, DMB-T, T-DMB
(Korea) CMMB (China), ISDB-T (Japan), ISDB-TB (Brazil),
Qualcomm’s MediaFLO™, MPH, DVB-H and, on the near horizon,
the DVB-T2 system. Along with the multitude of digital
modulation variants, the transmitter designer must not forget that
most of the world is still broadcasting analog television formats,
which include country-specific variants of PAL and NTSC systems.
Previous-generation exciters were mostly standard-specific and
used hardware-defined modulator circuitry or limited
software/firmware techniques, usually based upon the specific
standard. Recent advances in exciter design have allowed the use
of integrated circuits that can use software code to develop the
required modulation waveforms. Along with the capability to use
software-based modulation comes some very worthwhile
features: a) the ability to quickly load new software, allowing

migration from analog to any digital standard, or a change from
an existing standard to a new standard (e.g., from DVB-T to DVBT2) and b) the ability to automatically and continuously adjust
precorrection in the digital stages of the exciter, based on the
difference between the transmitted signal and the near-perfect
reference signal within the exciter. Such Real-Time Adaptive
Correction (for example, Harris RTAC™) provides the transmitter
network operator with a system that is simple to set up and
guarantees optimal performance at all times, even with varying
amplifier characteristics, fluctuating AC line voltages and thermal
drift of filter characteristics. By processing information from RF
samples both pre- and post-mask filter, the appropriate AM/AM or
AM/PM precorrection can be applied. An algorithm in the exciter
system constantly monitors the correction applied and makes
small adjustments, as needed, to keep the output as close to ideal
as is possible. Adaptive correction, such as RTAC, allows peace of
mind for the technicians, who no longer need to make such
corrections manually.
A simplified block diagram of the Harris software-defined exciter,
Apex™ M2X, is shown in Figure 1.

Figure 1 — Apex M2X Block Diagram

Power Level Requirements
A wide range of power levels is required to fulfill the varying needs
and applications of terrestrial broadcasters around the globe. A
few analog TV stations have transmitters with output powers of
100 kW or higher. On the other end of the scale, there are also
gap-filler, transposer and low-power transmitter requirements
that can be less than 10 Watts. As you can imagine, a 10,000:1
range in power level cannot easily be accommodated with one
design, or a single technology approach. At least two or three
basic architectures are typically required. These may be divided up
into air-cooled low power (typically 10 W to 1 kW average power),

air- or liquid-cooled medium to high power (1 kW to 20 kW
average power) and liquid-cooled very high power systems, often
employing efficient tube technologies. For the most frequently
used power range of 1 kW to about 10 kW average digital power,
there are solid-state devices that can readily fulfill the requirement,
providing important benefits — safety, simplicity and stability —
over earlier tube-type designs. Harris’ new Maxiva™ ULX
transmitter has been engineered for power levels from
approximately 1 kW average power to over 20 kW average power,
using the latest LDMOS FET device technology.

Transmitter System Architecture
A common approach that has been used for several years is shown below in the block diagram in Figure 2.

Figure 2 — Atlas Transmitter Architecture

This architecture uses basically two active RF components: the
exciter, which generates a low-level on-channel fully processed RF
signal; and an array of parallel, high-gain amplifier stages. Such
an approach has been used in recent Harris designs such as
DiamondCD® and the Atlas™ series of solid-state transmitters.
While this architecture provides excellent parallel redundancy and
on-air reliability, the drawback is that the PA stages tend to be
rather heavy and complex, and servicing requires skilled factory
technicians or well-trained station personnel with the proper test
fixtures. The high gain needed to take the low-level signal derived
from the input power divider forces the use of ALC or AGC
techniques, and fine adjustment is needed for overall module
phase and gain to ensure optimal module combining with least
loss. While such modules can be manufactured and are reliable,
they can prove to be rather daunting for the typical untrained user
when servicing is required.

A different approach that has already proven the test of time is the
use of lower-gain building blocks for the RF amplification. An
example of such architecture is the Harris® Platinum® series of TV
transmitters, developed in the late 1980s. Platinum transmitters
employed this type of low gain architecture, which resulted in
perhaps the most successful and most popular solid-state VHF-TV
transmitter ever designed, with over 1,200 units sold worldwide.
Reliability and ruggedness, along with ease of serviceability, were
the key attributes of the Platinum design.
A modern approach using this low gain building block approach
has been successfully implemented in the Maxiva ULX transmitter
architecture. Figure 3 shows the simplified block diagram of the
Maxiva ULX transmitter

Figure 3 — Maxiva ULX Transmitter Architecture

So as not to compromise reliability and on-air power levels during
servicing and replacement of the lower-level RF stages, parallel
redundancy with automatic switching is included as a standard
feature. In addition, each power module comprises smaller subassemblies, which enable the technician to replace an RF pallet, RF
device or AC-to-DC converters with ease and without the use of
expensive test equipment. With the use of an optional module
test unit, a simple test fixture can be provided for remote
diagnosis of the module away from the transmitter..
Other notable features include an all-new expandable control
system with built-in “life-support” control capability, which allows
basic operation if the main controller is faulty. Individual plug-in
cards are used for the various functions required and can be
replaced easily from the front of the unit. Standard parallel
remote control and Ethernet, Web GUI control are included as
standard features. In multiple-cabinet versions, individual cabinet
controllers are used to control each PA cabinet separately and
independently, providing even further system level reliability.

Power Density And Transmitter Footprint
Recent developments in LDMOS device technology have resulted
in major improvements in power density, resulting in more
compact transmitter designs. Solid-state designs from a few years
ago could achieve about 3.4 kW average COFDM power and 10

kW peak sync analog power per 19” rack cabinet. As newer RF
device technology has emerged, several manufacturers have taken
advantage of the higher per-package power levels of these
devices to develop transmitter power levels up to 5 kW to 7 kW
average power and up to 16 kW analog peak power. To provide
even higher power density, Harris, in partnership with a major
semiconductor supplier, has developed a design that provides
unsurpassed RF pallet power using newly developed state-of-theart LDMOS devices. Such devices are the core of Harris’ new
PowerSmart™ technology. These devices are the first UHF LDMOS
design to use a 50-volt structure, which results in an immediate
improvement in power per device and linearity/efficiency. The
devices are rated at 450 W CW power per package, which is far
superior to LDMOS devices used in previous-generation Harris
transmitters and current-generation transmitters from other
suppliers who use 150 W to 250 W power devices. An RF pallet
using a pair of the new devices can operate at approximately 180
Watts average DVB-T power — more than a 250 percent
improvement in power per pallet. In the Maxiva ULX design,
Harris elected to use four identical RF pallets per plug-in PA
module, resulting in a very compact and power-dense module
design. The overall PA module is rated at 650 Watts average DVBT power, which is significantly above the 460 Watts obtained form
the Atlas PA module that used twice the number of pallets per
module.

Another important feature of the new 50-volt LDMOS devices is
that the gain of each device is approximately 19 dB, a large boost
over standard 32-volt parts that typically offer 14 to 15 dB gain
per device. This increased gain works well with the Harris
architecture, due to the reduced number of driver stages required.
The Maxiva ULX PA component layout is shown in Figure 4.

combinations of PA modules to provide single-cabinet power
levels of up to 8.7 kW COFDM, 12.3 kW ATSC, or 26.2 kW
analog, providing the highest power density design currently
available from any major supplier. Multiple-cabinet configurations
can be used to provide even higher power levels, if required.
Figure 5 shows the simplified PA module block diagram.

The Maxiva ULX transmitter can be configured with various

Figure 5 â&#x20AC;&#x201D; Maxiva ULX PA Module Block Diagram

Efficiency And Long-Term Cost Of Ownership
While there are a multitude of factors that can affect long-term
cost of ownership, perhaps the most important and most often
misrepresented by suppliers is overall transmitter efficiency. For
digital transmitters, the efficiency can be calculated as the power
out divided by the power input. RF device data sheet efficiency at
CW is of little value to the customer. Actual circuit efficiency at

average digital power levels and usable crest factors are more
important. The new 50-volt devices used in the Maxiva ULX
transmitter can provide over 25 percent typical PA module
efficiency (AC power in versus RF power out), resulting in overall
transmitter efficiencies typically in the range of 20-22 percent.
When compared to previous designs, this represents an efficiency
improvement of up to 10 percentage points, or an improvement
of 35 percent or more from the original figure.